The invention relates to the manufacture of fabrics which possess a surface free of standing fibers or fiber loops. Such fabrics can be used as a support upon which a film or film-like material can be cast. The resulting composite will have neither fiber ends nor fiber loops protruding into or through the coating. The present invention is particularly well suited for use in the production of membrane filters, especially those for thin film composite (TFC) membrane filters.
In liquid filtration applications, such as reverse osmosis (RO) and ultrafiltration (UF), a membrane is often cast upon a support fabric. The fabric has many requirements including but not limited to strength, smoothness, and complete fiber tie-down of all surface fibers. In TFC technology, complete fiber tie-down is important for the supports upon which the thin film will be cast.
The coatings employed in TFC technology are of low viscosity, i.d., 300 to 400 cps and low add-ons (3 mils), compared to conventional methods (2000 cps and between 3 and 5 mils). Accordingly, membranes of very small thickness, i.e., less than three mils in many cases are produced. Because of the low viscosity coating and the fiber's rigid structure relative to the coating, any standing fibers will protrude through the coating creating a discontinuous membrane and a channel through which unfiltered liquid can bypass the membrane filter. Thus the filtered liquid may be contaminated.
Efforts to obtain suitable surface finishes with only a few standing fibers or fiber loops have involved using a hot calendering process with thermoplastic fibers, or a fabric made of spunbonded material (continuous fibers), or a combination of both. However, all these procedures produce fabrics which still possess standing fibers or fiber loops or involve relatively complicated manipulations rendering them less than completely satisfactory.
U.S. Pat. No. 3,837,995 discloses autogenously bonded multi-ply nonwoven constructions wherein one layer is composed of synthetic thermoplastic microfibers and another layer is composed of natural fibers. The layers of synthetic microfibers and natural fibers are bonded by contact only without need for supplemental bonding such as an adhesive or otherwise. Microfibers having a diameter of less than 10 microns and natural fibers such as wool, wood fibers, cotton, flax, jute and silk as well as those manufactured from natural polymer sources including rayon are utilized. The thermoplastic fibers used include polyolefins, polyesters, polyamides and polyacrylates. PG,5
U.S. Pat. Nos. 3,770,562 and 4,039,711 each relate to nonwoven fabrics wherein staple thermoplastic fibers are bonded to each face of a spun-bonded web of entangled and interlaced continuous filaments.
U.S. Pat. No. 4,180,611 discloses a nonwoven fabric adapted for use as a support material for a semipermeable membrane. The fabric is formed of a support mat into at least one surface of which an open-structured continuous covering layer of fine thermoplastic particles is calendered. The nonwoven fabric consists of a nonwoven hydrodynamically formed underlayer using thermoplastic staple fibers and has been preconsolidated by a drying process. Then a layer of fine thermoplastic particles is applied to the surface of the underlayer by electrostatic spraying of plastics from solvents. Layer weights of about 10 to 40 g/m.sup.2 are obtained.
U.S. Pat. No. 4,363,682 to Thiebault discloses a finishing process for smoothing the face of a nonwoven and highly aerated filtering layer and forming an electret by means of a metal mass at which the temperature is between 115.degree. and 150.degree. C. which passes over the surface to be smoothed.
Suzuki et al in U.S. Pat. No. 4,377,615 disclose nonwoven fabrics having an upper layer with a substantially smooth surface and a lower layer having a density lower than that of the upper layer. The lower layer contains a smaller amount of adhesive bonding products, i.e., 0.5 to 5 wt.-%, than in the upper layer, 15 to 40 Wt.-%. Each layer contains a distribution of hydrophobic and hydrophilic fibers. Polyester, polypropylene, acrylic, rayon, acetate and the like are used as the fiber in the upper and/or lower layer. Acrylic ester copolymer is used as adhesive. The composite can be used for preventing back flow of body exudates.